It has become understood that SCR performance will degrade over time, so that as an SCR ages, it is less efficient than it was when installed new in the vehicle exhaust system. In order to maintain required emission standards, it has become necessary to understand the aging process of the SCR and how to adapt the engine operation, particularly the urea dosing to a strategy that takes into account the age of the SCR.
When the SCR is operating at low temperature, ammonia is absorbed by the SCR, whereas at high temperatures, there is an increased ammonia slip past the SCR. At low temperatures, it is desirable to have a very high storage of ammonia in the SCR. At high temperatures, it is desirable to have low ammonia storage in the SCR. It has been determined that an SCR ages as a function of temperature of operation. It has been determined that the storage capacity of the SCR for ammonia degrades with SCR age. As the temperature of the SCR rises to about 500° C. or more, the performance degrades. Understanding the amount of time the SCR operates above a predetermined temperature can be used to map or populate a data table with expected levels of SCR efficiency, so that NOx and ammonia are not vented to the atmosphere, and so that a warning alert may be made to the vehicle operator once it is determined that the SCR is too old to be effective. Such information may be developed using a map or data points in a table. The map or data points may further be developed according to a one dimension model of the operation of the SCR and a one dimension model inverse logic model for the SCR. There is a need for a method to determine how urea dosing can be adjusted and the engine exhaust gas flow will meet emission standards regardless of the age of the SCR.